<template>
  <div>{{a.b}}</div>
</template>

<script>
export default {
  data() {
    return { a: {} };
  },
  created() {
    console.log(this);
    this.a.b = 1;
  },
  mounted() {
    this.a.b = 2;
    console.log(this.a.b);
  },
};
</script>